Servo actuators, which are hydraulically-powered driving devices, are very widely used. In some applications satisfactory performance is especially critical. For example, in helicopters, rotor position is controlled by two parallel servos, because that way gradual wear on the internal seals and other components in one actuator does not affect control characteristics and reliability. The redundancy also means, however, that gradual wear on one of the servos may go undetected: satisfactory operation of one servo "masks" the weakness of the other. The prospect of this "occult" servo degradation has been expensively addressed by disassembling the servo at periodic intervals and replacing worn-out parts, which also places the helicopter out-of-service for a period of time. A better approach to this problem is needed.